with this field, you assign the orders to concern employee… this is helpful when you have multiple guys in purchase / sales / operation dept and you want to assign the orders to one of them …later on you can restrict each of them to play with their assigned orders only…you need to select the employee manually …
during order creation you can assign the user responsible for that PO… or in your case you can do little bit of customization to auto update this field based on the (location code) values
then you can put a security filter on the role of warehouse guys to show only their assigned PO.